1. Field of the Invention
The present invention relates generally to the field of electric-powered vehicles and more particularly to managing energy usage from a secondary battery or energy source to a hybrid or totally electric vehicle, especially one that uses regenerative braking.
2. Description of the Prior Art
A major advance in the field of hybrid electric vehicles was made when Toyota Corporation introduced the Prius, an electric vehicle that also has an internal combustion engine and uses the technology of regenerative breaking to convert kinetic energy from braking back into electrical energy rather than just heat. The future of motor vehicles seems to be heading in the general direction of this vehicle. This type of vehicle is typically supplied with a single, high voltage, primary battery. In the case of the Prius, the battery is a nickel metal hydride battery (NiMH) supplying around 273.6 volts with 6.5 ampere-hour capability. Some models have slightly different voltages and capabilities (for example, 201.6 volts, 288 volts, 6.0 ampere-hours). The Prius electric drive motor has a power capability of around 67 Hp (depending upon the year and model). Vehicles of this type usually contain processors that coordinate the battery, power train and regenerative systems like regenerative breaking. Communication between various systems on the vehicle use a digital bus known as a CAN bus.
The primary battery pack is usually supplied with electronics that can keep track of stored energy, or state-of-charge, in the primary battery. A control system attempts to keep the battery pack around 72% of maximum (between 65%-75% typically). This is generally a compromise between storing as much energy as possible and leaving some room for energy from the regenerative braking.
FIG. 1 shows a typical prior art hybrid vehicle system. A high voltage battery pack supplies current to an electric motor. The motor may be DC or AC. If an AC motor is used, an inverter converts DC to AC for the motor. This motor drives the wheels when the vehicle is being operated electrically. A Battery electronics unit monitors the battery's state-of-charge and regulates charging and discharging. The battery is typically charged by the internal combustion engine (not shown) or by energy recovered in regenerative braking. A motor/generator is coupled to the wheels or drive train to act as a generator during braking. This unit can also act as a starter motor to start the internal combustion engine.
Because typically the primary battery pack is charged by the internal combustion engine, and because the vehicle only has a certain range in electric mode (EV mode), it can be very desirable to install a secondary battery pack in the vehicle. This way the vehicle can use energy stored in the secondary pack as well as the primary vastly extending the electric range of the vehicle. The secondary pack can be charged at home or business from 110 AC line service. This results in greater fuel economy since electricity is less expensive than gasoline per KW of usable energy.
There are two major difficulties in adding a second battery pack or other secondary energy source to this type of vehicle. The first is coercing the vehicle to use the available energy. Hybrid vehicles typically keep track of how much energy is stored in the primary battery. This is called the state-of-charge. If the state-of-charge is too low, then the internal combustion engine turns on to charge the battery. If the state-of-charge is too high, then the regenerative braking is disabled. In both cases, the advantages of hybrid technology is lost. Simply transferring energy from a secondary battery to the primary battery will cause the state-of-charge to reach levels that are too high to use regenerative braking. Hence braking energy will be unrecoverable. The second major difficulty with adding a secondary battery pack is metering energy from the secondary battery to the vehicle. Current control of some type is generally necessary to prevent large currents between the batteries.
There are systems known in the art for adding a second battery to a hybrid vehicle. Some are after-market. Various of these systems use a CAN bus monitoring device to decode vehicle communications and only transfer energy from the secondary battery when the state-of-charge is below some threshold. Some prior art systems transfer energy by having a secondary battery voltage that is higher than that of the primary battery and then connecting the packs together with a contactor. This approach may result in very high currents between the batteries that can result in battery overheating and even be a fire danger. Some prior art systems use a regulator circuit to adjust the voltage on the secondary battery to match that of the primary battery and hence limit the current. This approach allows the voltage of the secondary battery to be different from that of the primary battery, for example half. These methods almost exclusively require tapping into the CAN bus or other vehicle communications and then using the communications information to manage energy.
It would be advantageous to have a system and method of adding a secondary battery to an electric or hybrid vehicle like the Prius that would not require any interaction with the onboard vehicle communications system or any knowledge of the vehicle's controllers or processors. In this way, an external source-chargeable secondary battery pack could be added to extend the electric range of the vehicle and reduce the use of the internal combustion engine while maintaining battery currents and temperatures within safe ranges and allowing for the maximum use of regenerative energy systems like regenerative braking.